1. Field of the Invention
This invention relates to a process control device which controls a process; and more particularly, to such a device which functions to automatically adjust control constants, such as a proportional constant (P) and an integral constant (I), to optimum values; and still more particularly, to such a device having an internal process model to estimate the dynamic characteristics bf a process to be controlled, wherein PI operation parameters are determined by the process model based upon an estimated result.
2. Description of the Prior Art
In order to exert optimum control of a process, it is necessary to search the dynamic characteristics of the process and calculate the control operation parameters and use such parameters for calculation of the manipulated variables. These process control devices may before example, an open loop type, a limit sensitive type, a response waveform type, an adaptive control type, and a type equipped with a process model. FIGS. 1 to 5 are block diagrams depicting conventional process control devices.
FIG. 1 depicts a conventional process control device of the open loop type, wherein in a normal control state, switch 1 is connected to the control operation means side 2. Both set value SV and process variable PV are inputted to deviation operation means 3 wherein the deviation between the inputs is calculated. The deviation signal is inputted to control operation means 2 in which manipulated variable is calculated by using a PID operation parameter, and the obtained manipulated variable is outputted to process 4.
Switch 1 is periodically connected to identification signal generator side 5, or when the deviation becomes large, and a step signal, such as a pulse signal, is outputted from identification signal generator 5 to process 4 as an identification signal. Based on the response from process 4, control operation parameters are calculated in accordance with the Ziegler-Nichols method, etc, and set to control operation means 2.
FIG. 2 shows a conventional process control device of the limit sensitivity type, wherein a deviation signal from deviation operation means 3 is applied to control operation means 2 and limit cycle generator 6. Adding means 7 adds the outputs of control operation means 2 and limit cycle generator 6, and generates an output signal which is applied to process 4 as a manipulated variable.
Limit cycle generator 6 functions to change the manipulated variable outputted to process 4 to a maximum value and to a minimum value, periodically. Based upon the limit cycle or limit sensitivity at that time, the control operation parameters are calculated in accordance with the Ziegler-Nichols method, etc., and is set to control operation means 2.
FIG. 3 depicts a conventional process control device of the response waveform type, disclosed for example U.S. Pat. No. 4,602,326, wherein to response waveform rule applying means 8, a set value SV and a deviation DV are applied. Response waveform rule applying means 8 is configured by such tuning rules as Fuzzy rule, etc. Control operation parameters are obtained by applying these rules to the overshoot of the process quantity, the attenuation ratio, the attenuation period, etc, and the obtained parameters are set in control operation means 2.
FIG. 4 depicts a conventional process control device of the adaptive control type, wherein adding means 10 adds the output of control operation means 2 and the output of identification signal generator 9, and generates an output which is applied to process 4. Dynamic characteristic identification means 11 receives process variable PV from process 4 and the output from adding means 10, estimates control operation parameters, and sets the estimated parameters in control operation means 2. That is, a very small disturbance is generated in identification signal generator 9. The control operation parameter is obtained from the dynamic characteristic of the process variable PV at that time and the thus obtained result is set in control operation means 2.
FIG. 5 depicts a conventional process control device of the type equipped with a process model, such as disclosed in ISA Transaction Vol. 22, No. 3, p. 50,51; and U.S. Pat. No. 4,385,326, wherein a signal from control operation means 2 is outputted and applied to process 4 and also applied to process model 13 through a high pass filter 12. Process model 13, modelling process 4, is so configured that its parameters can be changed. Difference operation means 15 calculates the difference between the signal applied from process 4 through high pass filter 14 and the signal applied from process model 13. Model parameter search means 16 receives a difference signal from difference operation means 15, and searches parameters that make the difference signal minimal. The searched parameters are set as the parameters of process model 13. Based on the parameters searched by model parameter search means 16, control operation parameters are obtained, and the obtained control operation parameters are set in control operation means.
The above discussed conventional devices have a variety of problems. For example, the open loop device of FIG. 1 and the adaptive control device of FIG. 4 must provide identification signals to the process, and the limit sensitivity device of FIG. 2 must generate persistent oscillations. In these types of devices, disturbance is applied to the process. Thus, when the control constant is obtained while the process control device is in operation, many restrictions apply.
Moreover, in the response waveform device of FIG. 3, the waveform must be observed several times until the control operation parameters are settled, and hence, a certain time period is required.
Accordingly, in the foregoing control devices, the control operation parameters cannot be freely set.
Furthermore, in the device equipped with the process model, and shown in FIG. 5, the calculation is carried out continuously by model parameter search means 16. Thus, the amount of calculation increases as a whole. As a result, the load of a microprocessor used therewith becomes heavy when the function of each part is executed in accordance with a program installed in the microprocessor.